This invention relates to a pneumatic radial tire whose tread portion is reinforced by steel belt layers, and more particularly, to a pneumatic radial tire which exhibits an excellent belt durability though its steel belt layers use steel cords having a 1.times.2 twist structure.
Steel cords have a higher strength and a higher elastic modulus than other tire cords consisting of organic fibers. For this reason, the steel cords have found a wide application as reinforcing cords for belt layers for reinforcing the tread portion of radial tires. Conventionally, steel cords used for this steel belt layer have predominantly been of a 1.times.4 twist structure obtained by twisting four strands with one another and of a 1.times.5 twist structure obtained by twisting five strands with one another. However, as requirements for a further reduction of the weight of tires and the cost of the tire production have increased in recent years, a more simplified twist structure of steel cords for use for the steel belt layer than the conventional 1.times.4 twist structure and 1.times.5 twist structure has been increasingly sought.
The ultimate twist structure by the requirements described above is a 1.times.2 twist structure obtained by twisting only two strands with each other. According to this 1.times.2 twist structure, the number of strands is smaller, the weight can be reduced and the twisting step, too, can be simplified, so that the cost of production can be reduced, too. Furthermore, since this 1.times.2 twist structure permits high permeability by coating rubber, rust-proofness and separation resistance to rubber can be made better.
However, this 1.times.2 twist structure is the one in which two strands are merely twisted with each other. Therefore, the cross-sectional shape becomes elliptic as shown in FIG. 3 and the ratio of its maximum diameter a to its minimum diameter b becomes as great as 2:1. This means that a great difference occurs in the bending rigidity between when the steel cord is bent in the direction of the minimum diameter and when it is bent in the direction of the maximum diameter and when it is bent in the direction of the maximum diameter. If a steel belt layer is produced from the steel cords having such a great difference between the maximum diameter and the minimum diameter, steel cords which are adjacent to one another are likely to have same twist phases aligned with one another and there occurs a phenomenon in which portions having maximum diameters or portions having minimum diameters are aligned over a wide range in a direction crossing orthogonally the longitudinal direction of the steel cords when the steel belt layer is viewed on a plane. For example, FIG. 4 is an X-ray photograph of a steel belt layer produced by burying steel cords having a 1.times.2 twist structure of S twist into coating rubber. As can be seen, the portion in which maximum diameter portions are aligned with one another over a considerable width is found in a region A which is encompassed by chain lines among a plurality of steel cords 6 aligned parallel to one another.
If the same maximum diameter portions are aligned over a wide range in the steel belt layer, the flexural rigidity of this portion becomes extremely smaller than that of the portion where the minimum diameter portions are aligned, so that the out-plane flexural rigidity at this portion becomes extremely smaller than that at the other portion. Accordingly, when a large bending load is applied to a steel belt layer during heavy cornering or the like of a tire having such steel belt layer, the portion having a lowest rigidity, where the maximum diameter portions are aligned mutually, is easily bent and broken.
The inventors of the present invention examined in detail the steel belt layer comprising steel cords having the 1.times.2 twist structure and found out that the cause of the alignment of the twist phases between the adjacent steel cords described above results from the following phenomenon when the belt layer is produced from steel cords by calendering: a plurality of steel cords are passed through a die and are aligned in a reed-screen form at the time of calendering but the steel cords oscillate immediately ahead of the die and are in rotary motion with the longitudinal direction of the axis. When a plurality of steel cords are each in rotary motion in this manner, the rotations of the adjacent steel cords resonate and the twist phases are aligned. Moreover, since the difference between the maximum diameter and the minimum diameter of the sectional shape of the steel cord having the 1.times.2 twist structure is great, resonance of rotations is induced all the more easily and the twist phases are more likely to be aligned. Furthermore, since the steel cord having the 1.times.2 twist structure has a lower rigidity than the conventional steel cords having the 1.times.4 or 1.times.5 twist structure, an end count to be used for the steel belt layer must be made greater than that of the conventional steel belt layers. For this reason, resonance is induced all the more easily and the alignment of the twist phases is more likely to occur.